Gas-filled radiation detectors have been used for many years to provide qualitative and quantitative information concerning nuclear radiation. Such a detector consists of a hollow cathode defining a gas-filled chamber, and an anode within the chamber electrically insulated from the cathode. A voltage is applied between the anode and cathode. When the detector is placed in a radiation field, nuclear particles enter the chamber, causing ionization and the release of electrons. The ions and electrons are collected and characterized as to energy, type, numbers, etc. The results are typically viewed on an oscilloscope and are recorded and analyzed.
One type of detector is a Geiger-Mueller (GM) tube. A GM tube is characteristically operated in a high voltage range, thereby producing a large output signal which is independent of the nature of the initial ionizing event. Because of its extreme sensitivity, a GM tube can be used to detect all types of nuclear particles including beta, gamma and X-rays.
Geiger-Mueller tubes are presently used for a variety of purposes in research, medicine and industry. Among the varied uses are: detecting and recording particles emitted during experimentation on nuclear radiation; measuring the effect of bombardment on increasing the radioactivity of bombarded products; measuring and recording cosmic radiation; detecting and tracing radioactive substances in biological systems; using artificially activated substances to follow the progress of chemical and mechanical changes; and locating oil bearing strata in well logging. Furthermore, these tubes are used in such devices as oil level detectors or gauges on aircraft where they are subject to severe vibration and widely fluctuating temperatures, pressures and altitudes.
The chamber of a GM tube is filled with a monatomic and/or a diatomic gas which becomes ionized by radiation. Typically a noble gas such as neon or argon is also used. At the same time, a quench gas is used in the chamber to prevent the occurrence of unwanted secondary ionization caused by the release of electrons from the cathode. The quench gas has a lower ionization potential than the noble gas and dissociates to dissipate the excitation energy after pulsing.
Over the years, several quench gases have been used including organic gases such as ethyl alcohol, ethyl formate and methane, and inorganic halogen gases such as bromine and chlorine. The use of bromine is particularly advantageous because it has a very high electron capture cross-section and because its recombination rate after dissociation is nearly 100%. The temperature stability and long life of bromine are also outstanding. For these reasons, bromine quenched counters can be used continuously at temperatures of 300.degree. C. and for short intervals at temperatures as high as 400.degree. C.
A desirable attribute of a GM detector is high sensitivity or ability to detect low levels of ionization. To insure high sensitivity, the cathode is typically plated with an inert and dense (non-porous) layer of a metal such as platinum. Care must be taken to insure that the platinum is plated on the cathode as a coherent, non-porous layer. Among factors involved in insuring an adherent electroplate are the type of substrate surface used for the cathode, thoroughness of surface cleaning and preparation, characteristics of the plating bath, and plating conditions such as current density, temperature, presence or absence of bath impurities, etc. Deviation from optimum can lead to the formation of a porous deposit and attendant premature failure of the tube.
One criterion of performance of a GM tube is the uniformity of the count-rate over the entire span of operational voltages at which the count-rate is relatively independent of voltage. This stability persists over a wide range of operating temperatures. The stability plateau preferably occurs in the high voltage range thereby resulting in an improved pulse height and time resolution. The voltage approaches that value necessary to initiate spontaneous discharge between the conductive anode and cathode. A standard halogen quenched GM tube manufactured by The Harshaw Chemical Company typically exhibits a count-rate shift less than 2.5% and a slope of 8% over a range of 100 volts at a temperature ranging from -40.degree. C. to 225.degree. C. with an operating voltage range of 800 to 1000 volts.
In U.S. Pat. No. 3,342,538, I describe a method of increasing the maximum operating temperature of a GM tube. The method involves passing an electrical current through the tube in the presence of pure oxygen gas to oxidize the cathode surface. The oxygen is then replaced with a gaseous mixture containing halogen and the tube is thermally cycled at progressively higher temperatures to stabilize the oxide surface.
In U.S. Pat. No. 3,892,990, I describe a method of conditioning a GM tube for high temperature service without the necessity of thermally cycling the tube at progressively elevated temperatures. The improved characteristics of the tube are achieved by coating a stainless steel cathode with a thin layer of chromium, platinum or an alloy of nickel and copper. This is followed by passivation of the plated cathode by successively filling the tube chamber with halogen gas under pressure and purging the chamber until starting voltages are essentially constant after which a fresh charge of halogen gas is sealed in the chamber.
For most purposes, platinum is the preferred cathode coating because of its high sensitivity to gamma radiation. However, problems are encountered in the adhesion of platinum when it is plated on the inner surface of larger stainless steel tubes having a diameter of 1" or so. Another problem that is encountered with the very thin layer of platinum is its porosity. When using bromine or chlorine as a quench gas, the porosity of the platinum permits the free halogen in the gas to attack the stainless steel cathode. As the operating temperature increases, the rate of attack becomes even more pronounced. This causes the performance of the tube to degenerate with a drop in the starting voltage, a downward shift in the plateau and an attendant increase in the slope throughout the operating range of the counter. Because of the high cost of platinum, the economics surrounding its use are not favorable. Not to be overlooked is the considerable time and added expense of thermally cycling or passivating the platinum with bromine as described in my earlier patents.
Surprisingly, it has been found that a thin layer of tungsten can be used on the inside surface of a cylindrical cathode to give a GM detector having high sensitivity and outstanding resistance to the halogen gas.
Furthermore, it is surprising and unexpected that the tungsten layer can be applied to the interior of a cylindrical cathode as a thin foil thus omitting the necessity of electrodepositing the layer on the interior surface.
Of considerable significance is the fact that a stainless steel GM tube in which the interior surface of the cathode is coated with a thin layer of chromium oxide and a tungsten foil liner possesses remarkable high temperature stability even though it is not subjected to the thermal cycling or bromine passivation techniques described in my earlier patents.